1. Field of Invention
This invention is directed to correlation displacement transducers. In particular, this invention is directed to a correlation displacement transducer having a selectable detector area.
2. Description of Related Art
Various known measurement transducers use images acquired by a sensor array, and correlation between images acquired by the sensor array, to determine deformations and/or displacements. For example, one class of such devices is based on acquiring a speckle image generated by illuminating an optically rough surface with a light source. The light source may be a coherent light source, such as a laser-generating light source. Such laser-generating light sources include a laser, laser diode and the like. After the optically rough surface is illuminated by the light source, the light scattered from the optically rough surface is imaged onto an optical sensor. The optical sensor can be a charge-coupled device (CCD), a semiconductor image sensor array, such as a CMOS image sensor array, or the like.
Prior to displacing or deforming the optically rough surface, a first initial speckle image, sometimes called a reference image, is captured and stored. Then, after displacing or deforming the optically rough surface, a second or subsequent speckle image, sometimes called a current image, is captured and stored. Conventionally, as much of the first and second speckle images as possible are then correlated or compared on a pixel-by-pixel basis. In general, a plurality of comparisons are performed. In each comparison, the first and second speckle images are offset, or spatially translated, relative to each other. Between each comparison, the amount of offset, or spatial translation, is increased by a known amount, such as one image element, or pixel, or an integer number of image elements or pixels.
In each correlation or comparison, the image value of a particular pixel in the reference image is multiplied by, subtracted from, or otherwise mathematically used in a function with, the image value of the corresponding second image pixel, where the corresponding second image pixel is determined based on the amount of offset. The value resulting from each pixel-by-pixel operation is accumulated with values resulting from the operation performed on every other pixel of the images to determine a correlation value for that comparison between the first and second images. That correlation value is then, in effect, plotted against the offset amount, or spatial translation position, for that comparison, to determine a correlation function value point. The offset amount, or spatial translation position, having the strongest correlation between the reference and first images will generate an extremum, that is, a peak, or a trough, depending on how the pixel-by-pixel comparison is performed, in the plot of correlation function value points. The offset amount, or spatial translation position, corresponding to the peak or trough represents the amount of relative displacement or deformation between the first and second speckle images.
Similarly, conventional images may be used in image correlation displacement sensing system systems. For such applications, the light source may be an incoherent source, such as a light emitting diode (LED), and the scale or surface to be imaged is located at the object-side focal plane of the imaging optics used to image the pattern of the scale or surface onto the optical sensor of the correlation displacement transducer.